Among conventional metallic gaskets of this kind, there is a metallic gasket shown in FIG. 51 that is described in Japanese Patent Laid-Open No. 6-101761, for example. More specifically, this well-known metallic gasket is provided with a thickness-increased portion X formed at an inner peripheral portion of the combustion chamber opening 2 in a base plate 1 by folding back the peripheral portion on the combustion chamber opening 2 side of the base plate 1 or by mounting a shim plate 3 to the outer peripheral portion of the base plate and this gasket is also provided with a second thickness-increased portion C, which is thinner than the first thickness-increased portion X and formed by folding back an outer peripheral portion of the base plate. In addition, a rubber bead 4 is fixed on the outer side of the first thickness-increased portion X in a manner to protrude on both surfaces of the base plate and a rubber bead 5 lower in thickness than the thickness of the rubber bead 4 is fixed on the inner side of the second thickness-increased portion C in a manner to protrude on both surfaces of the base plate.
When the metallic gasket structured as described is sandwiched between the joint surfaces of the cylinder block and the cylinder head and fastened by clamping bolts, the rubber beads 4, 5 are compressed and deformed in the through-thickness direction, and at the end of fastening, because of the thickness difference between the first thickness-increased portion X of the largest thickness and the remaining portions of the base plate, surface pressure concentrates on the first thickness-increased portion and the largest load acts on it.
Therefore, applied to the joint is a fourfold seal which includes the surface pressure of the first thickness-increased portion X and the resilience of the rubber bead 4, and the surface pressure of the second thickness-increased portion C and the resilience of the rubber bead 5. Further, owing to the thickness-increase effect of the first thickness-increased portion X, the rubber bead 4 is prevented from totally collapsing and, also owing to the thickness-increase effect of the second thickness-increased portion C, the rubber bead 5 is prevented from totally collapsing.
As a second conventional metallic gasket, there is one which is described in Japanese Utility Model Application Laid-Open No. 63-180769, for example.
This metallic gasket is made by forming a base plate from a thin metal plate having rigidity, such as stainless steel plate, and various kinds of holes are formed in the base plate, such as an opening for the combustion chamber bore, bolt holes, oil holes. The holes, which require sealing, are enclosed by a full bead provided along the seal line. A repeated load by vibration amplitude caused by engine operation is applied to the metallic gasket, and in order to prevent fatigue failure of the bead by the repeated stress, the thickness-increased portions, formed to suppress the amount of bead deformation that occurs in the through-thickness direction, are provided along the whole internal peripheral portion on the combustion chamber opening side of the base plate.
With the progressive reductions of weight and size of the engine, the clearance between the adjacent combustion chamber openings is becoming smaller and the range in which beads and thickness-increased portions can be provided is becoming narrower in the boundary area between the adjacent combustion chamber openings. In view of this trend, in the prior art, a shared bead has come to be used in the boundary area, and furthermore, a thickness-increased portion with a minimum width is formed between the combustion chamber openings in the middle of this boundary area and a wide-width thickness-increased portion is formed in each end of the boundary area according to the clearance between the peripheral edges of the adjacent combustion chamber openings.
In order to secure a necessary sealing pressure by the bead which is elastically deformed in the through-thickness direction, a high-rigidity material, as mentioned above, is used for the conventional base plate.
Moreover, as the third conventional kind of metallic gasket, there is one which is disclosed in Japanese Patent Laid-Open No. 63-210465, for example.
In this metallic gasket, bead is a metal bead that is formed by bending the base plate (Refer to FIG. 52), and as shown in FIGS. 53 and 54, in a converge-diverge points M where seal lines 50 converge and diverge, the bead width becomes relatively wide, and after the seal lines 50 come together, they merge into a seal line 50 and the seal line 50 gradually becomes narrower.
There is another form of converge-diverge point which is described in Japanese Patent Laid-Open No. Hei 1-300043. More specifically, in this example, as shown in FIG. 55, a full bead which is protruded in one surface side in the form of mountain branches off into two half beads in stepped form, or, viewed differently, the two half beads converge into a full bead. This is a metal bead formed by bending a base plate.
Further, the fourth conventional type of metallic gasket is disclosed in Japanese Patent Laid-Open 2001-173791.
As shown in FIG. 59, this metallic gasket is formed by two base plates 50. More specifically, out of the two base plates 50, in the thicker base plate (the upper base plate), its end portion on the combustion chamber opening 51 side is bent to form a thickness-increased portion 52, and in each of the base plates 50, a convex bead 53 is formed on the outer side of the thickness-increased portion 52 so as to be higher than the thickness of the thickness-increased portion 52, and both base plates 50 are put together such that the convex portions of the base plates 53 facing each other. Moreover, the concave portions of the beads each facing outside are filled with an elastic sealing material 54.
When the above-described metallic gasket is sandwiched between the opposing joint surfaces of the cylinder block and the cylinder head and they are fastened by clamping bolts, the base-plate beads are compressed and deformed down to the thickness of the thickness-increased portion in the inner peripheral portion of the combustion chamber opening, and the elastic sealing material 54 filled in the concave portions is compressed and deformed to thereby seal the combustion gas, oil and cooling water pressure by a sealing pressure generated by a composite force including the spring force of the beads 53 made of the base plate and the resilience of the elastic sealing material 54. It goes without saying that there are conventional metallic gaskets without any elastic sealing material 53 filled in the concave portion and also there are conventional metallic beads made up of a single piece of base plate.
However, in the first conventional metallic gasket shown in FIG. 51, the rubber beads 4, 5 are provided on both surfaces of the base plate 1, and if the thickness of the folded-back portion of the thickness-increased portion is designated as t0, because the rubber beads 4 are provided on each surface of the base plate 1, the height of a rubber bead 4, for example, is t0/2+the compression-deformed amount ((t0/2) ≧0.4 (40% max.)). Assuming that t0 is 0.5 mm, the height of the bead 4 is 0.35 mm from the above equation. Thus, if the thickness of the base plate is thicker, it is possible to set a compression-deformed amount which sufficiently suits processing accuracy. However, if the thickness of the base plate is thinner, the compression-deformed amount is small so that it becomes difficult to process rubber beads, processing accuracy becomes stricter, thus increasing production cost.
Since water holes are normally formed in the base plate 1 between the rubber beads 4, 5, the rubber beads 4, 5 are exposed to cooling water, thus deteriorating their durability.
In the second conventional metallic gasket, a high-rigidity thin metal plate is adopted for the base plate to obtain a sufficient sealing pressure by elastic deformation of the beads, and therefore a thickness-increased structure is adopted to prevent fatigue failure of the beads, which is likely to occur by adoption of thin metal plate of high rigidity. However, the engines do not cease evolving and attempts have been made to reduce engine size and weight, improve performance and decrease fuel consumption, and as a result, there is a tendency toward higher combustion temperature and larger amplitude of vibration.
Therefore, even if a thickness-increased structure is adopted to prevent fatigue failure of the bead, as long as the conventional metallic gasket structures is used, there is a possibility that the bead suffers fatigue failure in a period of time shorter than a planned lifetime.
In the engines, a changeover to aluminum has been taking place for lightweight and better workability. When a cylinder block and a cylinder head are made by casting of aluminum, blowholes occur in the casting process, and while the joint surfaces are machined, blowholes may run through one to another, stretching across the seal line of the bead, giving rise to incomplete sealing somewhere in the joint.
Further, during plane processing of the joint surfaces of the engine, tool marks are left on the surfaces. To compensate for the tool marks, heat-resistant rubber with a thickness about twice the depth of the tool marks is applied to the gasket in a manner to cover the whole surface of the base plate with a rubber coating. If processing finish is rough, it is necessary to increase the thickness of the rubber coating, which will lead to a decrease in torque.
As has been described, the bead surfaces are covered with a thin rubber coating and the bead is so formed as to seal the joint by its rounded portion. Since the bead is formed by thin metal plate with high rigidity, it has a high resilience, and though the thickness-increase effect has been reduced, because internal stress concentrates in the above-mentioned rounded portion, when fatigue failure runs from one crack to another or the rubber coating is abraded off by vibration amplitude, the metal at the rounded portion of the gasket directly contacts the seal surface, which results in fretting or gas leak or fatigue failure.
Further, since the engines have been reduced in size and weight, the rigidity of the engines has decreased, unless the fastening axial tension is decreased, bore deformation will increase, which leads to greater oil consumption or power loss. For this reason, it appears that there is high demand for metallic gaskets which maintain sufficient sealing performance even if the fastening axial tension is made smaller than before.
In the third conventional metallic gasket, the bead seals the joint by metal contact and line contact by using a metal bead. Therefore, it is necessary to form the base plate, in other words, the bead by a material of high rigidity to increase the spring stress of the bead to secure a required sealing pressure.
More specifically, as shown in FIGS. 53, 54, it is inevitable that there are some partially wide parts at the converge-diverge points M of the seal line 50. As described above, this gasket has a structure to seal the joint only by the spring force of the metallic bead, and forms a line seal at three points (See A, B and C in FIG. 52) of the radius R of the bead, and at the converge-diverge point M where the bead width widens, as the span A-B becomes wide, the spring force becomes weak and sealing surface pressure becomes relatively low. However, it is impossible to increase the outer side radius R of the converge-diverge points M of the seal line 50.
Around the converge-diverge point M of the seal line 50, the bead has a relatively narrow width and forms a small radius R, with the result that the bead has a strong spring force close to that of a rigid body. Near the bead with strong spring force, there is locally formed a wide-width bead with a small spring force, in other words, partly with a low sealing pressure, and when a high pressure is applied to the wide-width bead, the bead is deformed partly, which results in leakage of pressure or liquid.
A metallic gasket with the above problem in mind is shown in FIGS. 56 and 57. With the bead of this metallic gasket, each of the seal lines that flow together into the converge-diverge point M is designed to keep its width as constant as possible. However, as described above, a high hardness material is used to provide high rigidity by which to generate a high spring force, for which reason the converging radius R of the line outside the converge-diverge point M cannot be made large, so that this gasket is nothing other than a minor improvement that is unable to obtain an equalized surface pressure because the above-mentioned radius R is small.
Among the conventional metallic gaskets is a type disclosed in Japanese Utility Model Laid-Open No. 5-42830. As shown in FIG. 58, this metallic gasket has a concave space formed in the middle of a wide-width converge-diverge point M to keep the bead width of the converge-diverge point M as constant as possible. However, this gasket has a shortcoming that the concave space in the middle is confined by seal lines, resulting in the converge-diverge point M having too strong a spring force.
As has been described, at the converge-diverge point M of the seal line 50, the bead width of the metal bead changes locally, and it is difficult to keep a uniform bead width. With the above-mentioned conventional metallic gasket, because it is necessary to set a large spring force for the bead, a high hardness material is used. Therefore, if the bead width changes as mentioned above, the spring force changes greatly. With the structure that parts of small radius R are inevitably formed as in the converge-diverge point M and that some parts exist where the spring force is large locally, supposing that the engine is made of aluminum, the sealing surfaces of the engine are prone to denting and scratching. If the engine sealing surfaces have local dented flaws, the flaws will lead to pressure leak when the gasket is changed. Even if the engine is made of cast iron, the converge-diverge point M is formed in a structure such that the surface pressure of the converge-diverge point M of the seal line 50 is high at the part of small radius R of the line outside the part M and that the part of a large radius R inside the part M is liable to oil leakage.
A base plate that has a larger number of converge-diverge points M requires a larger fastening force, and thus requires a larger total fastening axial tension.
When the rigidity of the region of the outer periphery of the base plate is weak, because of a large stress at the converge-diverge point M, the region of the outer periphery of the base plate is subject to a large deformation, which may lead to pressure leakage.
Besides on the converge-diverge part M, internal stress concentrates on the rounded portion of high hardness material, which may lead to fatigue failure by the amplitude of vibration and further lead to short lifetime.
In the fourth conventional metallic gasket (See FIG. 59), a resilience is generated jointly by the base-plate beads 53 and the elastic sealing material 54 filled in the concave portions when they are deformed by fastening, thus generating a required sealing pressure along the seal line.
However, if the base plates 50 are formed by metal plate of low hardness with a view to preventing fatigue failure of the base-plate beads 53 and reducing cost, in the above-mentioned conventional metallic gaskets, when bolts are fastened and the elastic sealing material 54 of the concave portion of the bead is compressed and deformed, an external force is applied such that the base plates 50 and the base-plate beads 53 are deformed in a manner to warp in the through-thickness direction. Since the base plates 50 are formed of metal of low hardness as mentioned above, the beads have a low shape-retaining force and accordingly the base plate 50 have an insufficient deformation-preventive force and hence a low sealing property.
By repeated load by repetition of operation and stoppage of the engine, after a long period of use, problems arise, such as a decrease in axial tension of the clamping bolts, changes with time of the base-plate bead 53 on the base plate 50, or deterioration in the elastic sealing material 54 of the concave portion of the bead; therefore, the sealing surface pressure is likely to drop. Such problems tend to manifest particularly at overhanging parts on the outer side of the clamping bolts.
When the elastic sealing material 54 is formed by baking in the concave portions of the base plates, even if the elastic sealing material 54 at high temperature is filled in the concave portions, it changes in volume by an amount of thermal expansion during subsequent open cooling, the center portion of the elastic sealing material 54 where the thickness is at its highest shrinks by an amount of thermal shrinkage. This is disadvantageous when the surface pressure decreases as described above. Such a phenomenon as this seems to be likely to occur particularly when the gasket is mounted in the engine which has been assembled with a weak fastening axial tension.